1. Field of Invention
The present invention relates generally to determining the position of an aircraft, and in particular to an apparatus and method for determining the position of an aircraft using time difference of arrival processing and range measurements.
2. Description of Related Art
The use of time difference of arrival techniques to determine the location of a transmitter is not unknown in the art. For example, U.S. Pat. No. 5,570,099 discloses a TDOA/FDOA technique for locating a transmitter. However, this technique requires the use of two or more separated radio frequency (xe2x80x9cRFxe2x80x9d) receiver terminals with large separation baselines for high accuracy. These receiver terminals also require specialized high gain antennas to produce adequate received signal strength for time difference of arrival (xe2x80x9cTDOAxe2x80x9d) calculations.
Typically, in order to produce high TDOA processing gain, the use of two or more highly stabilized clocks is required. These clocks are generally expensive atomic clocks and are local to each antenna receiver in order to accurately mark the time of arrival of the received signal.
Relative clock drift prevents the correlation of long blocks of data for high signal processing gain. Separate atomic clocks at each antenna receiver are used to reduce clock drift. Bringing RF signals together over long distances using RF coaxial cables or expensive wave-guides can be difficult. Coax attenuation of 500 dB per kilometer prevents a baseline separation of more than about 100 feet (30.48 meters). Secondary free space RF links cannot be used due to the interference with existing receivers.
Typically, high gain tracking antennas and atomic clocks for signal processing gain are required to achieve the sufficient signal strength for accurate TDOA calculations. These high gain antennas and atomic clocks tend to be expensive.
The present invention is directed to in a first aspect, a system for determining the position of an aircraft. The system comprises a first antenna, a second antenna, a processor coupled to the first and second antennas, and a position determinator. In the preferred embodiment, the first antenna is located at a first position, the second antenna is at a second position, and the baseline distance between the two antennas is known. The first and second antennas are adapted to receive an aircraft signal. The processor is adapted to calculate a time difference of arrival range between the second antenna and the aircraft using a common time reference signal. A position determinator determines the position of the aircraft based upon the baseline distance between the first antenna and the second antenna, the range between the first antenna and the aircraft, and the time difference of arrival range between the second antenna and the aircraft.
In another aspect, the present invention is directed to a method for determining the position of an aircraft. The method comprises positioning a first antenna at a spaced baseline distance from a second antenna and receiving an aircraft signal at each antenna. A first signal is sent from the first antenna, and a second signal from the second antenna, to a processor. The processor is preferably adapted to determine the time difference of arrival range between the second antenna and the aircraft using a sampled data set produced by time sampling the first signal and the second signal using a common time reference signal. Both first and second signals correspond to the aircraft signal. In the preferred embodiment, the position of the aircraft is determined by the baseline distance between the second antenna and the first antenna, the range between the first antenna and the aircraft, and the time difference of arrival range between the second antenna and the aircraft.
In another aspect, the present invention is directed to a method of using a time difference of arrival processor to determine the position of an aircraft transmitting a signal. The range (R1) between a second antenna and the aircraft is determined by measuring the range (R2) between a first antenna and the aircraft and calculating a time difference of arrival between the signal from the aircraft received by the second antenna and the signal from the aircraft received by the first antenna using a common time reference signal. The time difference of arrival may be adjusted for time slippage correlation delay and the speed of light. The position of the aircraft is determined by the baseline distance (R3) between the first antenna and the second antenna, the range (R2) between the first antenna and the aircraft and the range (R1) between the second antenna and the aircraft.